If all the concrete structures in America's 48 contiguous states were added up, they would cover a space almost as big as Ohio, researchers say.

Workers from several universities and agencies have put together the first ever map of the US, which shows "impervious surface areas" (ISA).

It is important to tot up concrete cover because of its harmful effect on the environment, the researchers claim.

The work was led by the National Oceanic and Atmospheric Administration.

Giant Jigsaw

If you made a giant jigsaw out of all the highways, streets, buildings, parking lots and other solid structures in the contiguous states, it would cover 112,610 sq km (43,480 sq miles). That is an area nearly the size of Ohio, which is 116,534 sq km (44,994 sq miles).

This is far more than a Christmas cracker statistic, the researchers claim, because concrete cover - or ISA - is not good for the environment.

The replacement of heavily vegetated areas by ISA reduces the depletion of carbon dioxide, which plants absorb from the atmosphere. This can speed up global warming.

ISAs can also alter the water cycle and disrupt aquatic ecosystems. They do this by changing the shape of stream channels, raising water temperatures and washing pollutants into aquatic environments.

Population growth

The ISA of the contiguous states is already slightly larger than that of its wetlands, which cover 98,460 sq km (38,020 sq miles).

The population of the US is increasing by three million a year. Concrete cover is spreading to match.

Every year, one million new family homes are built and 20,000 km (10,000 miles) of roads are laid.

Given these trends, it is likely a lot more will be made of impervious surface areas in the future.